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Elizabeth Coker and Terry Kaminski

The purpose of this study was to investigate the effects of visual condition (low light, full light, and full light with mirror) on balance control and technical form during two technical dance movements in a group of elite collegiate dance students. Dancers demonstrated higher center of pressure velocity indicating lower control while performing a static dance task (parallel relevé retiré) and a dynamic dance task (fondu relevé en croix) under low light conditions than either lighted condition. Measures of Western ballet technique (pelvic obliquity, knee extension, and ankle plantar flexion) showed no decrement under low light conditions. No effect of concurrent mirror feedback was found on either center of pressure velocity or technical requirements of the dance tasks.

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Daniel C. McFarland, Alexander G. Brynildsen and Katherine R. Saul

Most upper-extremity musculoskeletal models represent the glenohumeral joint with an inherently stable ball-and-socket, but the physiological joint requires active muscle coordination for stability. The authors evaluated sensitivity of common predicted outcomes (instability, net glenohumeral reaction force, and rotator cuff activations) to different implementations of active stabilizing mechanisms (constraining net joint reaction direction and incorporating normalized surface electromyography [EMG]). Both EMG and reaction force constraints successfully reduced joint instability. For flexion, incorporating any normalized surface EMG data reduced predicted instability by 54.8%, whereas incorporating any force constraint reduced predicted instability by 43.1%. Other outcomes were sensitive to EMG constraints, but not to force constraints. For flexion, incorporating normalized surface EMG data increased predicted magnitudes of joint reaction force and rotator cuff activations by 28.7% and 88.4%, respectively. Force constraints had no influence on these predicted outcomes for all tasks evaluated. More restrictive EMG constraints also tended to overconstrain the model, making it challenging to accurately track input kinematics. Therefore, force constraints may be a more robust choice when representing stability.

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Gregory S. Walsh, Daniel C. Low and Marco Arkesteijn

The aim of this study was to investigate the effects of backpack load carriage on quiet standing postural control and limits of stability (LOS) of older adults. Fourteen older adults (65 [6] y) performed quiet standing and a forward, right, and left LOS test in 3 conditions, unloaded, stable, and unstable backpack loads while activity of 4 leg muscles was recorded. Stable and unstable loads decreased postural sway (main effect ηp2=.84, stable P < .001, unstable P < .001), mediolateral (main effect ηp2=.49, stable P = .002, unstable P = .018) and anterior–posterior (main effect ηp2=.64, stable P < .001, unstable P = .001) fractal dimension, and LOS distance (main effect ηp2=.18, stable P = .011, unstable P = .046) compared with unloaded. Rectus femoris (main effect ηp2=.39, stable P = .001, unstable P = .010) and gastrocnemius (main effect ηp2=.30, unstable P = .027) activity increased in loaded conditions during LOS and quiet standing. Gastrocnemius–tibialis anterior coactivation was greater in unstable load than stable loaded quiet standing (main effect ηp2=.24, P = .040). These findings suggest older adults adopt conservative postural control strategies minimizing the need for postural corrections in loaded conditions. Reduced LOS may also increase fall risk when carrying a load. However, there was no difference between unstable and stable loads for postural control variables.

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Leila Ghamkhar, Somayeh Amiri Arimi and Amir H. Kahlaee

While proprioceptive functioning of the cervical extensor muscles has been proposed to be associated with chronic neck pain (NP), the mechanism by which such impairment might contribute to NP is not clear. The aim of this cross-sectional study was to investigate the relevance of proprioception, extensor muscles size, and endurance to chronic NP. A total of 60 participants with (n = 30) or without (n = 30) chronic NP participated in this cross-sectional study. Joint repositioning error (JRE), ultrasonographic parameters of the cervical extensors, and clinical extensor endurance were assessed. Multivariate analysis of variance and logistic regression tests were used to compare the groups and test the predictive value of the dependent variables for chronic NP, respectively. Patients showed comparable JRE scores but smaller multifidus size and lower extensor endurance (Cohen d = 0.66 for both). JRE in the transverse plane (β = 1.20), multifidus muscle size (β = 0.02), and endurance (β = 0.99) were significant predictors for chronic NP odds ratio. The results found multifidus size as the most relevant factor to NP by showing both between-groups difference and considerable odds ratio. As JRE in transverse plane was not different between the groups and extensor endurance demonstrated minimal odds ratio, these 2 factors may be considered as less relevant to NP comparing multifidus muscle size.

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J.D. DeFreese and Alan L. Smith

Person-centered investigations of athlete burnout have utility to unearth novel information about this developmental experience within the social environment of competitive sport. Guided by extant theory, conceptually proposed developmental patterns of athlete burnout were examined across a season as expressed in profiles of emotional and physical exhaustion, reduced accomplishment, and sport devaluation perceptions. Athlete social perceptions were also explored as predictors of profile membership. Collegiate athletes (N = 129) completed established assessments of study variables at four in-season time points. Latent profile analysis revealed profiles characterized by athletes experiencing the three burnout dimensions similarly at any given time point, with the notable exception of exhaustion being more frequently experienced in some profiles. Social support perceptions predicted profile membership with moderate success. Trends in profile stability provide some support for consideration of exhaustion-driven burnout experiences. Results shed light on the theoretical pathways of burnout development and inform continued longitudinal burnout research efforts.

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Mitchell Tillman and Satyajit Ambike

The authors examined how the stability of the current total isometric force (F T) produced by four fingers is influenced by previous and expected voluntary changes in F T. The authors employed the synergy index obtained from the across-trial uncontrolled manifold analysis to quantify the stability of F T. The authors compared two tasks with similar histories of F T changes; one in which participants expected changes in F T in the future, and one in which they expected no changes in F T. The stability of F T was lower in the former task, indicating the existence of a novel type of anticipatory synergy adjustment. Disparate histories of F T changes yield inconsistent changes in stability, driven by individual differences in the covariation in the finger forces that leave F T invariant. Future research should focus on exploring these individual differences to better understand how previous and expected behavior changes influence the stability of the current motor behavior.

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Jocelyn F. Hafer and Katherine A. Boyer

The link between age-related changes in muscle strength and gait is unclear. We tested if knee extensor functional demand differs by age and physical activity status and if functional demand increases with walking speed or after exercise. Gait and knee extensor muscle torque were collected from young adults and highly and less active older adults before and after treadmill walking. Functional demand was the ratio of knee moments during gait to knee extensor muscle torques estimated from participant-specific torque–velocity curves. Functional demand at the peak knee flexion moment was greater in less active older adults than young adults (29.3% [14.3%] vs 24.6% [12.1%]) and increased with walking speed (32.0% [13.9%] vs 22.8% [10.4%]). Functional demand at both knee extension moments increased ∼2% to 3% after exercise. The low functional demand found in this study suggests that healthy adults maintain a reserve of knee extensor strength.

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Hananeh Younesian, Nader Farahpour, Mehrdokht Mazde, Martin Simoneau and Katia Turcot

Diabetes peripheral neuropathy (DPN) leads to balance impairment among diabetes mellitus (DM). The aims of this study were to (1) distinguish between DM patients who have/do not have DPN and to (2) compare quadriceps’ strength and balance performance of DM, DPN, and healthy groups. Fifteen healthy females and 33 females with type 2 diabetic patients participated. The electrodiagnostic method was used to classify diabetic patients into DM and DPN. A dynamometer was used to measure quadriceps’ strength. Single-leg standing on a force plate was also used to quantify participants’ balance. Smaller conduction velocity and amplitude and greater distal latency of all nerves were observed in the DPN compared with the DM in particular for sensory nerve. In DPN, conduction velocity was asymmetrical. The quadriceps’ strength of both legs in DPN and the right leg in DM was smaller than in the control group. The root mean square of the center of pressure was similar between DM and DPN. But it was larger in DPN than in the control group. DPN is associated with asymmetrical conduction velocity, smaller quadriceps’ strength, and weaker balance performance that is suggestive of higher risk of falling. Balance training is recommended for the DPN group during their rehabilitation to reduce their falling risk.

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Nathaniel A. Bates, Nathan D. Schilaty, Ryo Ueno and Timothy E. Hewett

Anterior cruciate ligament (ACL) injury videos estimate that rupture occurs within 50 milliseconds of initial contact, but are limited by imprecise timing and nondirect data acquisition. The objective of this study was to precisely quantify the timing associated with ligament strain during simulated landing and injury events. The hypotheses tested were that the timing of peak strain following initial contact would differ between ligaments and that peak strain timing would be independent of the injury-risk profile emulated during simulated landing. A mechanical impact simulator was used to perform landing simulations based on various injury-risk profiles that were applied to each specimen in a block-randomized order. The ACL and medial collateral ligament were instrumented with strain gauges that recorded continuously. The data from 35 lower-extremity specimens were included for analysis. Analysis of variance and Kruskal–Wallis tests were used to determine the differences between timing and profiles. The mean time to peak strain was 53 (24) milliseconds for the ACL and 58 (35) milliseconds for the medial collateral ligament. The time to peak ACL strain ranged from 48 to 61 milliseconds, but the timing differences were not significant between profiles. Strain timing was independent of injury-risk profile. Noncontact ACL injuries are expected to occur between 0 and 61 milliseconds after initial contact. Both ligaments reached peak strain within the same time frame.

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Vagner D.O. Tavares, Kell G. da Costa, Daniel A.R. Cabral, Maria L.M. Rego, Menna Price and Eduardo B. Fontes

Impaired inhibitory control has been shown in individuals with substance use disorder (SUD). Cardiorespiratory fitness has been described as a potential factor to improve inhibitory control; however, the benefits in individuals with SUD are unclear. The aim of this study was to investigate the relationship between cardiorespiratory fitness with general and drug-specific inhibitory control in individuals with SUD. Sixty-two male participants under treatment for SUD performed a general and drug-specific inhibitory control test (go/no-go) and a cardiorespiratory fitness test. Cardiorespiratory fitness, age, and years of drug use were inversely associated with reaction time for both general and drug-specific inhibitory control. In addition, the regression models showed that cardiorespiratory fitness predicts general and drug-specific inhibitory control adjusted for age and time of drug use. However, cardiorespiratory fitness predicts equally both general and drug-specific inhibitory control. These findings suggest that increasing cardiorespiratory fitness could provide benefits in the inhibitory function of individuals with SUD.